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Convex Lens: Definition, Equation & Examples Video

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  • 0:00 Definition of a Lens
  • 1:01 Images Formed by Convex Lens
  • 1:48 The Thin Lens Equation
  • 3:47 Eye Disorders Treated…
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Convex lenses are used in eyeglasses, projectors, microscopes, telescopes, and many more optical instruments. In this lesson, learn what a convex lens is and what kinds of images it can form.

Definition of a Lens

Let's begin this lesson by getting a strong grasp of the general lens. A lens is an optical device that bends and focuses light. They're transparent pieces of glass or plastic with curved surfaces and the curvature of the surface determines how much and in what direction the light will bend. Lenses are able to bend light because of a phenomenon called refraction. Refraction occurs whenever light passes from one type of material into another, during which the light waves change speed and bend. This allows the lens to form images, and so lenses are used in all kinds of optical instruments, from eyeglasses to projectors to microscopes.

Lenses are classified based on the curvature of their surfaces. Those that curve inward are called concave, while those that curve outward are called convex. A convex lens is also known as a converging lens because it bends light towards a single focal point.

Images Formed by Convex Lens

Convex lenses can form two types of images: real and virtual. A real image is one that is projected in front of the lens and can be captured on a screen. Convex lenses are the only type of lens that can form real images. If you've ever seen a movie in a theater, then you've seen a real image created by a convex lens. Projectors, like those used in movie theaters, use convex lenses to project real images onto screens.

Convex lenses can also form virtual images, the kind you have to look into the lens to see. Many people use eyeglasses to see clearly, and these often have convex lenses. If you're wearing glasses with convex lenses, you are always looking into the lens, so everything you see is actually a virtual image created by your lenses.

The Thin Lens Equation

You can calculate the type of image formed by a convex lens by using the thin lens equation. In this equation, S1 is the distance from the object to the lens, f is the focal length of the lens, and S2 is the distance from the object to the image.

thin lens equation

The thin lens equation can be used with all types of lenses, not just convex lenses. Note that when the equation is applied to a convex lens, the focal length will always be a positive number. Let's look at an example of how to use the thin lens equation.

First, let's take a lens with a focal length of 0.15 m and place an object 0.2 m in front of it. What type of image will be formed and where will the image be located? Use the thin lens equation to find the image distance (S2):

thin lens example 1

What does it mean to have an image distance of +0.6 m? Since the image distance is positive, the image is real and on the opposite side of the lens from the object. This is the type of image that could be projected on a screen. What would it mean if the image distance (S2) were negative? In that case, the image would be virtual, and you would have to look into the lens to see it.

We can also use this information to find out something about how big the image will be. Once we know the image distance and object distance, we can calculate the magnification (M), which tells you how much bigger or smaller the image is than the object.

MAGNIFICATION

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